This invention relates generally to arc lamps primarily but not exclusively for use as projection lamps in motion picture projectors.
Xenon arc lamps generate significant heat in operation and must be cooled in order to achieve acceptable lamp life. In an air-cooled projector, the lamp is located in a lamphouse and sufficient air flow must be provided within the lamphouse to remove heat generated by the arc lamp. An example of a projector having an air cooled xenon arc lamp is disclosed in U.S. Pat. No. 5,587,750 (Gibbon, et al.). The patent does not provide specific disclosure of how the lamp is cooled but FIG. 1 of the drawings does show (at 46) a hose through which cooling air for the lamphouse (42) is exhausted from the projector. The disclosure of the Gibbon et al. patent is incorporated herein by reference.
A xenon arc lamp typically has a glass envelope enclosing an anode and a cathode between which the arc is struck. An atmosphere of inert xenon gas under pressure is provided within the envelope. The anode and cathode are located in a bulb in the glass envelope at opposite ends of respective electrode assemblies. The electrode assemblies are housed within coaxial cylindrical portions of the envelope that extend in opposite directions from the bulb. Accordingly, the lamp has a defined axis represented by the anode and cathode assemblies. In some applications, the lamp is oriented with its axis vertical (e.g. xc2x115xc2x0), usually with the anode uppermost. Xenon arc lamps can, however, be run in a horizontal orientation also.
The patents literature contains numerous examples of proposals for cooling lamps. A xenon arc lamp with improved reflector cooling is disclosed in U.S. Pat. No. 5,721,465 (Roberts). A searchlight incorporating a xenon arc lamp is disclosed in U.S. Pat. No. 5,369,557 (Ronney).
U.S. Pat. No. 4,630,182 (Moroi et al.) discloses a prior proposal for cooling short arc mercury lamps. Unlike xenon arc lamps, a short arc mercury lamp does not have an anode and cathode, and the orientation of the lamp in operation is not critical.
Examples of other patents that disclose inventions relating to the cooling of lamps are U.S. Pat. Nos. 5,091,835 (Malek, et al.) and 5,458,505 (Prager).
The present inventors have recognized that, in an air-cooled arc lamp, while sufficient air flow must be provided to remove heat generated by the lamp, the nature of the air flow over the bulb of the lamp also is important and can affect the performance of the lamp. If the air flow is too great, gas (xenon) turbulence can be created within the bulb itself, causing arc instability. In the case of a projection lamp, this instability can be seen as an annoying flicker on the projection screen. It has been found that the air flow can also contribute to arc instability and flicker if it is non-uniform over the surface of the lamp bulb.
Where the lamp is oriented vertically (usually anode upwards) it has been found that the arc has a tendency to wander at high frequencies, which is especially noticeable as flicker on the projection screen. For this reason, it has been recognized as critical to precisely control the air flow over the lamp.
Accordingly, the present invention is aimed at addressing these problems both as they relate to arc lamps for motion picture projectors, and in arc lamps generally.
In one aspect of the invention, there is provided a method of cooling an arc lamp having coaxial anode and cathode end portions and a glass envelope that includes a bulb between said end portions. The method involves supporting one of the end portions of the lamp in a lamphouse by means of a support that includes a shroud for that end portion of the lamp. The shroud provides an annular air space around the end portion of the lamp, and has an inlet for cooling air and an annular air outlet that is directed towards the bulb of the envelope. Cooling air is caused to flow through the shroud from the shroud inlet to the shroud outlet. Air leaving the outlet flows over the bulb as an annular airstream, cooling the lamp.
The invention also provides an arc lamp assembly that includes a lamp of the form referred to previously and a lamphouse including a light collector and an opening through which light reflected from the lamp by the collector leaves the lamphouse. The lamphouse has an inlet and an outlet for cooling air, and a fan is provided for causing air flow between the inlet and the outlet. The assembly includes lamp supports for the respective end portions of the lamp for positioning the lamp appropriately with respect to the collector. One of the lamp supports includes a shroud for the relevant end portion of the lamp, that provides an annular air space around that end portion of the lamp. The shroud has an inlet for cooling air and an annular air outlet that is directed towards the bulb of the lamp envelope. The cooling air inlet to the lamphouse communicates with the air inlet to the shroud and the cooling air outlet from the lamphouse is located remote from the shroud so that cooling air flows through the shroud in use and leaves the shroud outlet as an annular airstream that flows over the bulb for cooling the lamp.
As indicated previously, the invention is based on the recognition that precise control of cooling air flow over the surface of the lamp is critical to arc stability. The annular air gap between shroud and the end portion of the lamp (usually the anode end) creates a xe2x80x9csheetxe2x80x9d of laminar air flow which tends to xe2x80x9cadherexe2x80x9d to the surface of the bulb, thereby providing precise cooling. In practice, it has been found that only one cooling fan is necessary for a projection lamphouse and the that air flow can be precisely controlled to provide adequate cooling while avoiding excessive air flow (which could lead to unwanted arc movement).
Preferably, the shroud itself is carried by a support arm that extends inwardly from a wall of the lamphouse. The arm is hollow and communicates with the air inlet to the shroud at one end, and with the air inlet to the lamphouse at its opposite end. The cooling air then flows along the arm and into the shroud. The shroud preferably is positioned asymmetrically with respect to the end of the lamp so that the gap between the shroud and the lamp varies from a maximum adjacent the air inlet to a minimum at the opposite side of the end of the lamp. This offset addresses the tendency of air to move faster near the inlet. The wider gap reduces the air speed while the narrower gap at the opposite side increases the speed of the air. Overall, the result is a more uniform air flow around the entire circumference of the lamp.